Method for controlling nox concentration in exhaust gas in combustion facility using pulverized coal

ABSTRACT

An object of the invention is to provide a method for controlling an NOx concentration in an exhaust gas in a combustion facility that uses a pulverized coal, which can easily control the NOx concentration in the exhaust gas to be discharged from a fuel facility that uses the pulverized coal as the fuel to or below a regulation value according to the Air Pollution Control Law and the like, and can also reduce an amount of a denitrifying agent or the like to be used, which is necessary for the control, by controlling the NOx concentration on the basis of the properties of the pulverized coal beforehand. The invention further includes: measuring a reaction velocity of each of chars corresponding to a plurality of types of pulverized coals beforehand; determining a relationship between the NOx concentration in the exhaust gas and the reaction velocity in advance; blending the plurality of the types of the pulverized coals so that the reaction velocity of the char becomes such a value as to correspond to a target NOx concentration or below, on the basis of the relationship; and supplying the blended pulverized coal to the combustion facility as the fuel of the combustion facility.

TECHNICAL FIELD

The present invention relates to a method for controlling an NOxconcentration in an exhaust gas to be discharged from various combustionfacilities which use a pulverized coal as a fuel.

BACKGROUND ART

Generally, in a calciner in a cement manufacturing process, a pulverizedcoal is used as a fuel for heating and calcining a cement material inthe calciner.

As is illustrated in FIG. 3, the cement manufacturing facility providedwith this calciner includes: a rotary kiln 1 for burning a cementmaterial; a preheater 3 provided with a plurality of cyclones 4 a to 4 dwhich are provided on a kiln inlet part 2 of this rotary kiln 1 in theleft side of the figure; a chute 5 for feeding the cement material fromthe cyclone 4 d in the lowermost stage of this preheater 3 to the kilninlet part 2 of the rotary kiln 1; an exhaust line 9 having an exhaustfan 10 which is connected to the cyclone 4 a in the uppermost stage anddischarges a combustion exhaust gas; a main burner 7 for heating theinner part of the rotary kiln, which is provided at a kiln outlet part 6in a right side in the figure; a clinker cooler 8 for cooling a cementclinker that has been burnt, which is provided at the kiln outlet part6; and further a calciner 12 having the lower end to which thecombustion exhaust gas is introduced from the kiln inlet part 2 of therotary kiln 1 through a duct portion and also having a combustion deviceof the pulverized coal provided therein which is fed from a not-shownfuel feed line, between the cyclone 4 c in the third stage and thecyclone 4 d in the fourth stage. For information, there is also a cementmanufacturing facility having the calciner 12 provided in another pathwhich is different from the duct portion on the kiln inlet part 2.

In a cement-clinker manufacturing facility having the above describedstructure, the above described cement material which has been fed to thecyclone 4 a in the uppermost stage shall be preheated by ahigh-temperature exhaust gas which is sent from the rotary kiln 1 andascends from the lower part, as the cement material falls downsequentially to the cyclones 4 in the lower part, then be extracted fromthe cyclone 4 c, be sent to the calciner 12, be calcined in the calciner12, and then be introduced into the kiln inlet part 2 of the rotary kiln1 from the cyclone 4 d in the lowermost stage through the chute 5.

On the other hand, the combustion exhaust gas which has been dischargedfrom the rotary kiln 1 shall be sent to the cyclone 4 d in the lowermoststage through the calciner 12, be sequentially sent to the cyclones 4 inthe upper part to preheat the above described cement material, andfinally be exhausted by the exhaust fan 10 from the upper part of thecyclone 4 a in the uppermost stage through the exhaust line 9.

By the way, in such a cement manufacturing facility, the concentrationof nitrogen oxides (hereinafter referred to as NOx) in the exhaust gasto be discharged from the exhaust line 9 by the exhaust fan 10 isregulated by the Air Pollution Control Law. For this reason, the cementmanufacturing facility monitors the NOx concentration in the combustionexhaust gas at all times and controls the NOx concentration in theexhaust gas so that the concentration does not exceed the abovedescribed regulation value, by removing the NOx by appropriatelyspraying ammonia water, sludge containing ammonia water or the like intothe exhaust line 9, or by lowering the temperature in the rotary kiln 1by adjusting the amount of fuel of the main burner 7, as are describedin the following Patent Literatures 1 and 2, and the like.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. H10-194800-   Patent Literature 2: Japanese Patent Laid-Open No. H10-192896-   Patent Literature 3: Japanese Patent Laid-Open No. H05-196212-   Patent Literature 4: Japanese Patent Laid-Open No. H07-310903

SUMMARY OF INVENTION Problems to be Solved

For this reason, when the NOx concentration in the above describedexhaust gas becomes high, the amount of the sprayed ammonia water,sludge or the like is increased, and a white smoke and a foreign odormight occur. In addition, when the temperature in the rotary kiln 1 isexcessively lowered by the adjustment for the amount of the fuel of themain burner 7, a quantity of heat necessary for burning the cementmaterial decreases, and such an operation problem might occur as thedeterioration of the quality and the reduction in a quantity ofproduction.

On the other hand, it is known that the NOx generated by the combustionof fuel is divided roughly into NOx (Thermal-NOx) originating in heat,and NOx (Fuel-NOx) originating in fuel.

Then, in order to solve the above described conventional problems, asfor the NOx originating in the heat, it is investigated to use a low-NOxburner or the like, as is proposed in the above described PatentLiteratures 3 and 4.

In addition, as for the NOx originating in the fuel, attention is paidto a nitrogen mass ratio in the fuel or a fuel ratio (=fixed carbonratio/volatile component ratio), and a method is adopted which lowersthe NOx concentration by using a fuel having a lower nitrogen mass ratioor by increasing a combustion speed with the use of a fuel having asmall fuel ratio (having large ratio of volatile components). However,any method has such problems that estimation accuracy is low andaccordingly it is difficult to control the NOx concentration into adesired NOx concentration range in the operation. Then, it has beenexpected to improve the problems.

Then, the present inventors have made the following examination andstudy in order to control the NOx concentration in the exhaust gas whichis discharged particularly from the above described calcining furnace12, with high accuracy.

Firstly, a process in the above described calciner 12 which uses thepulverized coal as the fuel is a process of calcining the cementmaterial, and a temperature in the calciner is 800 to 900° C.Accordingly, it is considered that the NOx in the exhaust gas to bedischarged from the calciner 12 is mainly NOx originating in the fuel.

Then, the present inventors measured properties (nitrogen mass ratio,fuel ratio and reaction velocity of char (fixed carbon)) of each ofpulverized coals with respect to a plurality of types of pulverizedcoals which were used in the above described calciner 12, and alsomeasured the NOx concentration in the exhaust gas which was dischargedfrom the above described calcining furnace 12 in the case where asubstance obtained by blending the above described pulverized coals wasactually used as fuel. Subsequently, the present inventors calculatedthe properties (nitrogen mass ratio, fuel ratio and reaction velocity ofchar (fixed carbon)) of the pulverized coal which was actually used asthe fuel, on the basis of the above described blend ratio, and examinedan influence of these properties on a change of the NOx concentration inthe above described exhaust gas.

As a result, as illustrated in FIG. 4A, a determinate correlationshipcould not be found between a nitrogen mass ratio (N wt. %) in the usedpulverized coal and the value of the NOx concentration in the exhaustgas in the outlet of the calciner. In addition, similarly, a significantcorrelationship could not be found also between the fuel ratio and thevalue of the NOx concentration in the exhaust gas in the outlet of thecalciner, as illustrated in FIG. 4B.

From these results, it was found difficult to expect the desireddecrease of the NOx concentration or to control the NOx concentrationwith such an accuracy as to be necessary in the operation, by only usinga fuel having a low nitrogen mass ratio or using a fuel having a smallfuel ratio, as in the above described conventional technology.

Then, the present inventors estimated or measured the reaction velocityof the char on the basis of the blend ratio of the above describedpulverized coals, and compared the estimated or measured reactionvelocity to the change of the NOx concentration in the exhaust gas inthe outlet of the calcining furnace. As a result, a strongercorrelationship was found between the reaction velocity and the changeof the NOx concentration than that between the above described nitrogenmass ratio and the change of the NOx concentration or that between thefuel ratio and the change of the NOx concentration, and specificallysuch a tendency was found that the NOx concentration in the exhaust gasin the outlet of the calciner decreased as the reaction velocity of thechar is high (FIG. 5).

The present invention has been designed on the basis of the abovedescribed finding, and an object of the present invention is to providea method for controlling an NOx concentration in an exhaust gas in acombustion facility using pulverized coal, which can easily control theNOx concentration in the exhaust gas to be discharged from thecombustion facility that uses the pulverized coal as the fuel, to orbelow a regulation value according to the Air Pollution Control Law andthe like, and can also reduce an amount of a denitrifying agent or thelike to be used, which is necessary for the control, by controlling theNOx concentration on the basis of the properties of the pulverized coalbeforehand.

Means to Solve the Problems

In order to solve the above described problems, a method for controllingan NOx concentration in an exhaust gas to be discharged from acombustion facility that uses a pulverized coal as fuel, which isdescribed in claim 1, includes: measuring a reaction velocity of each ofchars corresponding to a plurality of types of pulverized coalsbeforehand; determining a relationship between the NOx concentration inthe exhaust gas and the reaction velocity in advance; also blending theplurality of the types of the pulverized coals so that the reactionvelocity of the char becomes such a value as to correspond to a targetNOx concentration or below, on the basis of the relationship; andsupplying the blended pulverized coal to the combustion facility as thefuel of the combustion facility.

In addition, the invention according to claim 2 includes using afrequency factor of the reaction velocity in place of the reactionvelocity of the char, in the invention according to claim 1.

Furthermore, the invention according to claim 3 is the method in theinvention according to claim 2, wherein the frequency factor of reactionof char is determined by: drawing a curve of a time change of weightloss for the plurality of the types of the pulverized coals under thecondition of a plurality of temperatures by using a thermal balance;dividing the inclination of a tangent of the curve by a measured partialpressure of oxygen; thereby determining the reaction velocity of thechar in the temperature; and subsequently drawing the Arrhenius plotwhich sets (1/measurement temperature (unit: absolute temperature)) forthe horizontal axis and sets the value of the reaction velocity of thechar in the measurement temperature for the vertical axis; anddetermining the frequency factor of the reaction of the char from anintercept of the vertical axis of the Arrhenius plot.

Advantageous Effects of Invention

As is clear from the experimental result by the present inventors, whichwill be described later, the method according to the invention describedin any one of claims 1 to 3 can easily control the NOx concentration inthe exhaust gas to be discharged from the combustion facility, to orbelow a regulation value according to the Air Pollution Control Law andthe like, by measuring the reaction velocity of each of the charscorresponding to a plurality of types of pulverized coals which are usedin a combustion facility beforehand, measuring the NOx concentration inthe exhaust gas corresponding to the used pulverized coal in advance,and setting the value of the reaction velocity of the char so that theNOx concentration becomes a target value or below, on the basis of theserelationships, when the plurality of the types of the pulverized coalsare blended.

In addition, even when the value of the reaction velocity of the abovedescribed char cannot be controlled so that the NOx concentration in theexhaust gas becomes the target NOx concentration or below, only by theabove described blend of the pulverized coal, the method according tothe present invention can more greatly reduce the amount of thedenitrifying agent or the like to be used, which is separately added forreducing the NOx concentration, than a conventional method, bydecreasing the NOx value as much as possible by the adjustment of thereaction velocity, and accordingly can reduce also the occurrence of anoperational harmful effect originating in the addition of thedenitrifying agent or the like.

Incidentally, the reaction velocity k (1/s·Pa) of the char of the abovedescribed pulverized coal can be expressed by the following expression(1).

k=A·exp(−E/RT)  (1)

Here, “A” represents a reaction frequency factor (1/s·Pa) of char, “E”represents an activation energy (J/mol), “R” represents a gas constant(8,314 J/K·mol), and “T” represents an absolute temperature (K).

Accordingly, the reaction velocity k of char itself is a function of atemperature T. Accordingly, in order to control the NOx concentration inthe exhaust gas on the basis of the reaction velocity k of char of eachpulverized coal, it is necessary to consider also the factor of thetemperature T, which makes the operation complicated.

Then, the present inventors drew the Arrhenius plot which sets themeasurement temperature for the horizontal axis and sets the abovedescribed reaction velocity k of char in the measurement temperature forthe vertical axis when having measured the above described reactionvelocity k of char by using a thermal balance, determined the abovedescribed activation energy E from the inclination of the Arrheniusplot, and determined the above described reaction frequency factor A ofchar from the intercept of the above described vertical axis. As aresult, it was found that the above described activation energy E couldbe considered to be a constant value (inclination was constant) in eachof the pulverized coals and the difference among those was within 10%.

As a result, as illustrated in FIG. 5, it was proved that there was alsoa similar strong correlationship between the change of the reactionfrequency factor A of char and the change of the NOx concentration inthe exhaust gas.

Accordingly, when the NOx concentration is controlled with the use ofthe above described reaction frequency factor A of char as arepresentative value of the properties of the pulverized coal, in placeof the above described reaction velocity k of char, as in the inventionaccording to claim 2, it is not necessary to consider the influence ofthe temperature in the calciner when the pulverized coal is usedtherein, which accordingly further facilitates the control of thepulverized coal and the combustion control.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph schematically illustrating a method of measuring areaction velocity of char and determining a reaction frequency factor ofchar from the Arrhenius plot which has been drawn on the basis of theresult of the measurement, in one embodiment of the present invention.

FIG. 2 is a graph illustrating a relationship between the reactionfrequency factor of char, which has been determined in the abovedescribed embodiment, and a necessary amount of ammonia water to beused.

FIG. 3 is a schematic block diagram illustrating a cement manufacturingfacility to which the above described embodiment has been applied.

FIG. 4A is a graph illustrating a relationship between a nitrogen massratio of the pulverized coal and an NOx concentration in an exhaust gas.

FIG. 4B is a graph illustrating a relationship between a fuel ratio ofthe pulverized coal and the NOx concentration in the exhaust gas.

FIG. 5 is a graph illustrating a relationship between the reactionfrequency factor of char of the pulverized coal and the NOxconcentration in the exhaust gas, which has been determined in thepresent embodiment.

EMBODIMENTS OF THE INVENTION

One embodiment will be described below in which a method according tothe present invention for controlling an NOx concentration in an exhaustgas in a combustion facility using pulverized coal, on the basis of anexperimental example that has been conducted by the present inventors,has been applied to the control of the concentration of NOx originatingin the combustion of the pulverized coal in a calciner 12 in the cementmanufacturing facility illustrated in FIG. 3.

Firstly, as illustrated in FIG. 1, a reaction velocity k of charcorresponding to each of a plurality of types of pulverized coals whichwere used in the above described calciner 12 was measured with the useof a thermal balance.

Incidentally, the used thermal balance is an infrared differential typedifferential thermal balance TG8120 made by Rigaku Corporation, and aweight of a sample was measured with an electronic balance XS105DU madeby Mettler-Toledo International Inc. In addition, a powder was used ofwhich the 50% cumulative diameter was 10 to 40 μm when measured with alaser diffraction method, as the sample which were used for theevaluation of combustion properties.

The mass reductions of samples were measured according to a process of:raising the temperature of a predetermined amount of the sample of thepulverized coal in an atmosphere of nitrogen gas at a rate of 15 K/s;after the temperature has reached a predetermined temperature, holdingthe temperature until the change of the weight loss due to the thermaldecomposition of a volatile component contained in the above describedsample became sufficiently small (for 1 to 4 minutes); then switchingthe atmosphere of nitrogen gas to an atmosphere containing oxygen; andholding the sample at a plurality of constant temperatures (K).

Incidentally, the plurality of the constant temperatures and the amountsof the samples when the mass reductions of the above described sampleswere measured were 500° C. (1.5 mg), 550° C. (1.0 mg), 600° C. (0.5 mg),650° C. (0.2 mg) and 700° C. (0.1 mg), respectively.

At this time, a time change (reaction rate change dX/dt) of the weightloss due to the oxidization of the char in the above described thermalbalance can be expressed by the following expression (2).

dX/dt=k(1−X)n P _(O2) m  (2)

Here, k represents a reaction velocity (1/s·Pa) of char, X represents areaction rate of char, P_(O2) represents a partial pressure (Pa) ofoxygen, and m and n represent orders of the reaction.

If the above described orders of the reaction have been determined to ben=0 and m=1 (which are determined by another prediction experiment),k=(dX/dt)/P_(O2), and accordingly a value obtained by dividing theinclinations of tangents in the time change of the weight loss in thecases of unburned ratios of 0.3, 0.5 and 0.7 (though in the figure, thecase of 0.5 is illustrated) by the measured partial pressure of oxygenshall be the reaction velocity k of char at the temperature T.

Next, the above described measurement and the like were conducted ateach of the above described temperatures (K), and the Arrhenius plot wasdrawn which sets 1/T (1/K) for the horizontal axis and sets the reactionvelocity k of char for the vertical axis. Then, the above describedactivation energy E was determined from the inclination of the Arrheniusplot and the reaction frequency factor A of char was determined from theintercept of the vertical axis, for each of the unburned ratios 0.3, 0.5and 0.7 (though in the figure, the case of 0.5 is illustrated). Then,the average value of the reaction frequency factors A of char, which hadbeen determined for each of the unburned ratios 0.3, 0.5 and 0.7, wasdetermined to be the frequency factor A of the measured sample.

Subsequently, a relationship was determined between the reactionfrequency factor A of char of the pulverized coal which was actuallyused in the calciner 12 and the NOx concentration in the exhaust gasdischarged at that time. FIG. 5 is a graph illustrating thisrelationship. It is understood that there is a strong correlationshipbetween the frequency factor and the NOx concentration.

In addition, it is understood that in order to control the NOxconcentration to 500 ppm or below which is an allowable range in theoperation of the above described calciner 12, the pulverized coal may beblended so that the reaction frequency factor A of char becomes 15 ormore.

Furthermore, when ammonia water or the like is finally added to a gasincluding an exhaust gas to be discharged from the rotary kiln 1, theamount of ammonia water or the like to be used can be predicted as isillustrated in FIG. 2.

Thus, the method for controlling the NOx concentration in the exhaustgas in the calciner 12 which has the above described structure and usesthe pulverized coal therein can easily control the NOx concentration inthe exhaust gas to be discharged from the calciner 12 to such a value asto be within an allowable range in the operation of the calciner 12, byblending a plurality of types of the pulverized coals which are used inthe calciner 12 so that the reaction frequency factor of char of theblend becomes the value of the reaction frequency factor A of char atwhich the NOx concentration becomes the target value or below, on thebasis of the relationship between the reaction frequency factor A ofchar and the NOx concentration, which is illustrated in FIG. 5.Incidentally, the reaction velocity k of char and the reaction frequencyfactor A after the blending can be practically determined withoutproblems by the weighted average of each value of the blended pulverizedcoals in consideration of the blend ratio, in addition to actualmeasurement.

In addition, the method can prevent also such an operational problemfrom occurring that the quality is lowered and a quantity of productionis reduced, which originates in excessively lowering the temperature inthe rotary kiln 1 so as to lower the NOx concentration in the exhaustgas that is discharged from the exhaust line 9, by controlling the NOxconcentration in the calciner 12 to be within the allowable range in theoperation. In addition, it also becomes possible to reduce the amount ofammonia water which is finally added to a gas including the exhaust gasfrom the rotary kiln 1, in the exhaust line 9.

INDUSTRIAL APPLICABILITY

The method of controlling an NOx concentration in the exhaust gas to bedischarged from a fuel facility which uses a pulverized coal as a fuel,on the basis of the properties of the pulverized coal beforehand, caneasily control the NOx concentration to or below a regulation valueaccording to the Air Pollution Control Law and the like, and can alsoreduce an amount of the denitrifying agent or the like to be used, whichis necessary for the control.

EXPLANATION OF REFERENCE NUMERALS

-   -   12 calciner (combustion facility)

1. A method for controlling an NOx concentration, in an exhaust gas in acombustion facility that uses a pulverized coal as a fuel, comprising:measuring a reaction velocity of each of chars corresponding to aplurality of types of pulverized coals beforehand; determining arelationship between the NOx concentration in the exhaust gas and thereaction velocity in advance; blending the plurality of the types of thepulverized coals so that the reaction velocity of the char becomes sucha value as to correspond to a target NOx concentration or below, on thebasis of the relationship; and supplying the blended pulverized coal tothe combustion facility as the fuel of the combustion facility.
 2. Themethod for controlling the NOx concentration in the exhaust gas in thecombustion facility which uses the pulverized coal according to claim 1,comprising using a frequency factor of the reaction velocity in place ofthe reaction velocity of the char.
 3. The method for controlling the NOxconcentration in the exhaust gas in the combustion facility which usesthe pulverized coal according to claim 2, wherein the frequency factorof reaction of the char is determined by: drawing a curve of a timechange of weight loss for the plurality of the types of the pulverizedcoals under the condition of a plurality of temperatures by using athermal balance; dividing the inclination of a tangent of the curve by ameasured partial pressure of oxygen; thereby determining the reactionvelocity of the char in the temperature; subsequently drawing theArrhenius plot which sets (1/measurement temperature) for the horizontalaxis and sets a value of the reaction velocity of the char in themeasurement temperature for the vertical axis; and determining thefrequency factor of the reaction of the char from an intercept of thevertical axis of the Arrhenius plot.